There aren't any built-in settings or pre-existing apps to display upload and download speeds in the Ubuntu 11.04 panel, but there is an easy way to do it using a combination of a system monitoring applet, dstat and a short script.
You can use a panel applet called System Monitor Indicator to display realtime upload speed, download speed, memory and CPU usage. It wouldn't normally display the upload/download speed data, but you can change that by using a short script and a tool called dstat.
It works in both Ubuntu 11.04 and 10.10, whether you're using the full Unity interface or the slimmed-down Ubuntu Classic.
This quick and easy hack was put together by the folks at WebUpd8, who wrote the script that makes it possible. Hit the link below for step-by-step instructions.